1. Field of the Invention
The present invention relates to a method for the soiling check of the window of a measuring apparatus for checking sheet material, a measuring apparatus which is formed for carrying out the method, and an apparatus for processing sheet material, which contains the measuring apparatus.
2. Description of Prior Art
Apparatuses for processing sheet material are apparatuses such as processing machines for value documents, for example bank note processing machines for checking bank notes, or other processing machines such as for example cash depositing machines and cash dispensing machines. There can be checked sheet material of any kind, in particular value documents, such as for example bank notes. In general, “sheet material” is to be understood to be sheet-shaped objects. With value documents there is the need to check them for authenticity, and value documents such as bank notes that are heavily used and become soiled thereby must be occasionally checked for their condition and if the soiling is too heavy withdrawn from circulation. For this reason, bank note processing machines check the bank notes for authenticity, but, partly, also for their condition, in particular for their soiling condition. Too heavily soiled bank notes are sorted out by the bank note processing machine and, where applicable, automatically destroyed in the machine, so that they are removed from circulation.
Since a preferred field of application of the present invention are bank note processing machines and the check of bank notes, the invention is described hereinafter with reference to bank note processing machines and measuring apparatus for checking bank notes. The present invention is by no means limited to this field of application, however, but can be used for the check of any sheet material.
In apparatuses for checking bank notes, the bank notes are usually irradiated on one or on both surfaces by means of at least one light source, and the remitted light and/or the transmitted light is detected by means of suitable optical sensors. For the protection of the sensors, between the transported bank notes and the sensors there are mostly windows that are transmissive for the employed wavelengths, in order to prevent a mechanical damage or a soiling of the sensors. In the course of time, however, these windows become soiled, so that the remitted light, which is detected by a sensor, not necessarily comes from the checked bank note, but possibly from a soiling of the measurement window. The result of the optical check of the bank notes may be distorted thereby. For example, the soiling, upon viewing along the transport direction of the bank note through the processing machine, leads to bright streaks on the detected bank note image. These streaks come about e.g. by scattered light of the locally soiled measurement window. The measurement window or the measurement windows are therefore cleaned, when a certain degree of soiling has been reached.
For ascertaining when a cleaning is necessary, the soiling of the measurement windows is regularly automatically checked during the operation of a bank note processing machine. For this purpose, the sensor detects, in the phases when no bank note is in its capture region, the remitted or the transmitted light. The established value is compared with a reference value for a clean window. When the difference exceeds a specified value, the need for cleaning is indicated or a cleaning is automatically carried out, for example through blown air directed onto the measurement windows. Alternatively, the measurement windows can be manually cleaned. Since the operation of the bank note processing machine usually must be interrupted for the cleaning, frequent manual or automatic cleaning work leads to a reduction of the bank-note throughput of the machine